Device for converting a saw-toothshaped voltage of a comparatively low frequency into a saw-toothshaped current



Jan. 18, 1949. J. HAANTJES 2,459,273

DEVICE FOR CONVERTING A SAW TOOTH-SHAPED VOLTAGE OF A COMPARATIVELY LOW FREQUENCY INTO A SAW TOOTH-SHAPED CURRENT Filed July 16, 1946 DEF LECTING 2 CO! LS g to FIGJZB JOHAN HAANTJES INVENTOR AGENT Patented Jan. 18, 1949 UNITED STATES PATENT OFFICE DEVICE FOR CONVERTING'A SAW-TOOTH- SHAPED VOLTAGE OF A COMPARATIVELY LOW FREQUENCY INTO A SAW-TOOTH- SHAPED CURRENT ApplicationJuly 16, 1946, Serial No. 684,081 In the Netherlands March 20, 1942 Section 1, Public Law 690,;August s, 1946 Patent expires March 20, 1962 3 Claims. 1

This invention relates to a circuit for converting a sawtooth-shaped voltage of comparatively low frequency into a sawtooth-shaped current in the deflecting coils of a cathode-ray tube preferably serving for the image deflection in a television transmitting or receiving device.

As is well-known, this conversion may be effected by means of a discharge tube having a high internal resistance, for example a pentode, the control grid having supplied to it the sawtoothshaped voltage to be converted and having in- 'cluded in its output circuit the deflectingcoils.

Sincethe internal resistance of the discharge tubeishigh relatively to the impedance of the deflecting coils the latter are traversed by a sawtooth-shaped current.

In order to prevent the direct-current component of this current from passing through the deflection coils and bringing about an undesirable initial deflection of the luminous spot on the screen of the cathode-ray tube. it is usual to place a condenser in series with the deflecting coils and to connect in parallel with this series-connection a choke coil through which the anode voltage is supplied to the discharge tube. In thiscase the alternating-current component of the sawtoothshaped output current of the discharge-tube flows through thedeflecting coils, while at least-part of thfi direct anode current flows through the choke col One form ofconstruction of such a circuit is shownin Fig. 1, which is a diagrammatic view. Figures 2a and 2b are graphs illustrating the current as a function of time in the circuit.

In this form of construction the image deflectingcoils of a cathode ray tube of which the resistance amounts to T2 and the inductance Lz; are included in the output circuit of a pentode, 2, the grid circuit of which has supplied to it a sawtooth-shaped voltage originating from a source of supply'E. "The deflecting coils have connected in parallel to them a choke-coil having a resist ance nand an' inductance Lrand through which flows the direct anode current of the tube V. A condenser C, which constitutes a short cir'cuit for the alternating-current component of the output current of the tube V, prevents the direct anode current'from' flowing through the deflecting coils.

' In the described circuit a current 1" having an exact'sawtooth shape, which issho'wn'inFig. 2a

2 as a function of time, flows in the anode circuit of the discharge tube V. The direct-current component of this current flows through the choke coil, whereas a sawtooth-shaped alternating current flows through the deflecting coils.

Due to the presence of the choke-coil, however, the current passing through the deflection coils will no longer exhibit an exact sawtooth-shape, which leads to disturbances in the movement of the electron beams of the cathode-ray tube. The variation of the current flowing through the deflection coils is represented in Fig. 2b as a function of time by the drawn line, whereas the dotted line indicates the ideal variation of the current flowing through the deflecting coils.

A comparison of the deformed curve with the ideal curve demonstrates that the deformation substantially occurs during that part of the sawtooth-shaped current which is active for scanning the screen of the cathode ray tube.

It is already known that the said deformation may be suppressed by proportioning the parallelconnected choke coil in such manner that the re sistance to inductance ratio of the choke coil is equal to the resistance to inductance ratio of the deflecting coils.

In practise this condition for proportioning the choke coil involves the'following drawback. v The resistance inductance ratio of a deflecting coil is rather great, in particular if coils with iron core are used so that, if the said condition is fulfilled; the impedance of the circuit parallel-connected to the deflecting coils is substantially determined by the resistance. Since it is desirable, however, to reduce this resistance as much as possible in View of the direct voltage loss which oecurs across the resistance, the impedance of the parallel circuit will under these conditions be compartively low, due to Which a considerable part of the sawtooth-shaped output current of the tube V flows'through this circuit and the arrangement has a bad efficiency.

Further it is known with the use of a choke coil having a high self-inductance that substantially no deviation from linearity occurs. However, the

use of a choke having a high self-induction entails the drawback that the resistance of the choke coil is also high, a high direct voltage loss occurring across the latter.

The present invention has for its purpose to provide the minimum value for the choke coil connected in parallel with the deflecting coils, the percentage deviation from linearity during the active part of the sawtooth-shaped current not exceeding a permissible value of about According to the invention the choke coil is so proportioned as to be approximately equal to 101'2T, in which n represents the resistance of the deflecting coils and T the period of the sawtoothshaped current.

The elfect of the invention may be demonstrated as follows, it being taken that the flyback time of the sawtooth-shaped voltage suppliedto the grid of tube 2 and consequently that of the sawtooth-shaped current flowing in the anode circuit of tube V is negligible relatively to the period. This condition is practically always satisfled at low frequencies, in particular with the sawtooth generator in an image deflecting circuit.

The output current of the tube V, which exhibits the shape of curve represented in Fig. 2a, periodically increases in a time interval comprised between i=0 and i=1 linearly with time and consequently that part of the current which is variable With time may be represented by i at in which a is a constant factor. 'It may be demonstrated by calculation that that part of the periodical sawtooth-shaped current i which periodically flows through the deflecting coils in a time interval comprised between i=0 and t=T, may be represented by T-FF' l 2 is a constant factor which is determined by the ratio of the resistances T1 and'rz and s T1+T2 id-L2 is the ratio of the sum of the resistances n and T2 to the sum of the self-inductances L1 and L2,

The curvature of the current is is determined by the term T j ;(P-q) The above-stated relation for 2 may consequently be replaced by:

and

in which 4 represents a constant part which is invariable with time.

The steepness of the curve giving the variation of i2 is represented by:

increases with time and at the time t=T has attained the maximum value 0:8(17-(1) T.

During the time interval comprised between i=0 and t=T the largest relative deviation a is consequently determined by the time t=T the steepness y I dt may have at most a relative deviation a of approximately 10% from the ideal sawtooth-shaped curve, this leads to the relation have the relation With the aid of this relation the minimum value for the choke coil at which the deformation still remains within permissible limits may be determined. I

I claim:

1. A device for converting a sawtooth-shaped voltage of comparatively low frequency into a sawtooth-shaped current in the deflecting coils of a cathode ray tube preferably serving for the image deflection, the sawtooth-shaped voltage being supplied to a discharge tube having a high internal resistance whose output circuit includes the deflecting coils which have connected in parallel with them a choke-coil through which flows at least part of the direct-current component of the sawtooth-shaped output current of the dis; charge tube, in which device the inductance of this choke coil is approximately equal to l0rzT ,'Tz representing the resistance of the deflecting coils and T the period of the sawtooth-shaped current.

' 2. A circuit for converting a relatively low frequency sawtooth voltage into a sawtooth current in the deflecting coils of a cathoderay tube, said circuit comprising an electron discharge device including a cathode, agrid' and an anode, said device having a high internal resistance, means to apply thesawtooth'voltage asan input tov thegrid of said device, means to couple thedeflecting coils between the anode and cathode of said device, and

a choke coil coupled between the anode and cathode of said device, at least a portion of the directcurrent component of the sawtooth output current of said device flowing through said choke coil, the inductance of said choke coil being approximately equal to 10rzT, wherein n represents the resistance of the deflecting coils and T the period of the sawtooth current.

3. A circuit for converting a relatively low frequency sawtooth voltage into a sawtooth current in the deflecting coils of a cathode ray tube, said circuit comprising a pentode electron discharge device including a cathode, a grid and an anode,

a source of anode potential, a coupling capacitor, means to apply the sawtooth voltage to said grid, means to couple th deflecting coils between said anode and said cathode in series with said capacitor and said source, and a choke coil connected between said anode and the junction of said source and said capacitor, the inductance of said choke coil being approximately equal to IOrZT, wherein n represents the resistance of the deflecting coils l0 and T the period of the sawtooth current.

J OHAN HAANTJES.

No references cited. 

